Saturable absorber (SA) is a critical component for ultrafast lasers which tunes the continuous wave output into a train of ultrafast optical pulses. The key requirements for a saturable absorber are its wavelength range (where it absorbs), its dynamic response (how fast it recovers), and its saturation intensity and fluence (at what intensity or pulse energy it saturates). They are commonly used for passive Q-switching.
Currently, the dominant technology is based on semiconductor saturable absorber mirrors (SESAMs). However, these have a narrow tuning range, and require complex fabrication and packaging. A simple, cost-effective alternative is to use single-wall carbon nanotubes (SWCNTs), where the diameter controls the gap and thus the operating wavelength. Broadband tunability is possible using SWNTs with wide diameter distribution but the non-uniform chiral properties of SWCNTs present problems for precise control of the properties of the saturated absorber. SWCNTs that are not in resonance can cause insertion losses when operating at a particular wavelength.
Therefore, there remains a need for tunable saturable absorbers with cheaper fabrication, lower power consumption and multi-wavelength operation for applications in ultra-fast laser systems and novel types of plasmonics.